Regarding Angle of Attack Indicators

[old man emu]

A couple of months ago I started this thread: https://www.recreationalflying.com/topic/37152-learning-about-lift-generation-a-waste-of-time/ and in it I said " So all a pilot needs to know to maintain lift is to keep the angle of the chord line to relative airflow direction below 15 degrees." I followed that up with this: So, how does a pilot maintain that angle? In the majority of aircraft that the average private pilot would fly, there is nothing to use as a visual reference to compare the chord line with the relative wind direction. The obvious tool would be something like a bubble level set on the instrument panel when the aircraft was trestled in its cruise position, and calibrated to show position pitch angle from 0 to 20 degrees with reference to the chord angle.

 

This hypothesis started a flurry of comment which I tried to answer by describing how an indicator could be made by adapting the simple indicator that is usually used to show slip. That description seemed not to be understood. My original intention was to suggest a device that could be used during "normal" operations such as take off or level flight at low forward speed (eg while doing ground searches). There resulted many posts describing unusual flight attitudes that were beyond the scope of my hypothesis.

 

I went off in search of examples where similar devices had been used. I found that before gyroscopic attitude indicators became the norm, bubble type levels had been used.

Note the final paragraph.

 

I also found that a similar device was used on the pre-WWI Bleriot XI aircraft. 

 

And finally something similar was used in the Tiger Moth. That's the white thing with the red line.

 

Knowing that the Shuttleworth Collection had the oldest flying original Bleriot XI, I wrote to them for information about the use of the device, and this is the reply I got.

 

Hi,

This type of longitudinal clinometer (I sent a picture of the triangular one - OME) was used primarily in instrument flying as an indication of whether the aircraft was in a climb or decent attitude. We don’t have one on our Bleriot and I’m surprised to see that a pre-war aircraft would have one. This is because you don’t need one when you have a visible horizon - it’s much easier to see the position of the aircraft’s nose or the cabane struts relative to the horizon than look at a little instrument. But if they did do some cloud flying it would help and it could have been an aid to setting the right attitude. It may have been a  bit of gimmick or an aid to students - I don’t know.

 

The triangular glass tube is continuous and half filled with a coloured liquid (normally long since dried up) therefore if the instrument is held level, the bottom half of the tube will be half filled with the coloured liquid with airspace above - there is no bubble as such (unless you consider the airspace above the fluid as being one large bubble). Normally it is mounted in the instrument panel with just the graduated quadrant showing and the point of the triangle is facing forwards. As the aircraft (relative to the earth) points nose down the fluid moves down the glass and vice versa. This one appears to be made to mount on the sidewall of the cockpit with the point facing aft.

 

The instrument has to be used with much caution as it only senses ‘g’ in effect or force acting on the liquid if you prefer. That force can come from gravity or any other ‘g’ force/acceleration experienced by the aircraft. For example an aircraft may be flying in a level attitude bodily speaking in relation to the earth but still descending towards it. It can’t tell AoA either- it has no way of knowing where the relative wind is coming from.  An aircraft can stall at any attitude, even pointing straight down.

 

However, it can give an indication of decent or climb and you could derive AoA but in each case it would only be for a given set of parameters (My emphasis - OME). For practical purposes then, in steady/unaccelerated flight, at a given speed airspeed the instrument can be set at the right angle to give the pilot an indication of whether he is flying level or at a descending or climbing attitude (providing there are no significant up or downdrafts of course).

 

In unaccelerated, dead level flight, it could also be used give an indication of the AOA but that is of very little practical purpose.

 

Gyroscopic attitude indicators soon replaced these instruments for obvious reasons.

I hope this helps.

 

John Munn

Chief Engineer (and Collection pilot).

 

So, to sum up and end the discussion, my hypothesis that a bubble level, suitably fitted with reference to the angle between the longitudinal axis of the aircraft and the chord line of the wing, and with markings adjusted so that the zero point indicated the normal straight and level AoA of the wing, would work. However, and this brings the discussion to a close, as, in the words of John Munn, such an indicator is of very little practical purpose. 

 

I think that all participants in this discussion can accept that assessment.

 

[pmccarthy]

This is worth a read https://drive.google.com/file/d/0B1WyIFBvIVXGaVdfRkFGM0UtYjA/view

 

[old man emu]

1 hour ago, pmccarthy said:

This is worth a read https://drive.google.com/file/d/0B1WyIFBvIVXGaVdfRkFGM0UtYjA/view

 

Yes. A good paper and to my mind clearly identifies the most difficult task - to identify the direction of airflow relative to the chord.

 

In my simple hypothesis I didn't take into account any other direction of the airflow than that parallel to the longitudinal axis of the aircraft, and by extension, the direction of the airflow relative to the chord. I just looked at the simplest scenario of obtaining enough Lift to maintain a desired altitude as airspeed decreased - the "straight ahead stall" situation. Having read the paper, I realise that, apart from a straight ahead climb out, or slow level flight in search conditions, there are other "normal" stages of flight - climbing turns and side-slips - where the direction of airflow is not parallel to the longitudinal axis of the aircraft. Since I was looking at non-aerobatic flight, I'll ignore high speed stalls when pulling out of dives.

 

As Mr Munn from Shuttleworth put it, you could derive AoA but in each case it would only be for a given set of parameters. So, my idea is not a final solution, but 

[facthunter]

And if you are going in the wrong direction you have to get back to where you started before you get anywhere.. Nev